function checkForm(formId){
  form=document.getElementById(formId);
  ret=true;
  
  if(form.vorname.value==""){
    form.vorname.style.backgroundColor='red';
    ret=false;
  }else{
    form.vorname.style.backgroundColor='green';
  }
  
  if(form.nachname.value==""){
    form.nachname.style.backgroundColor='red';
    ret=false;
  }else{
    form.nachname.style.backgroundColor='green';
  }
  
  if(form.strasse.value==""){
    form.strasse.style.backgroundColor='red';
    ret=false;
  }else{
    form.strasse.style.backgroundColor='green';
  }
  
  if(form.plzort.value==""){
    form.plzort.style.backgroundColor='red';
    ret=false;
  }else{
    form.plzort.style.backgroundColor='green';
  }
  
  if(form.telefon.value==""){
    form.telefon.style.backgroundColor='red';
    ret=false;
  }else{
    form.telefon.style.backgroundColor='green';
  }
  
  if(form.mail.value==""){
    form.mail.style.backgroundColor='red';
    ret=false;
  }else{
    form.mail.style.backgroundColor='green';
  }
  
  return ret;
}